Re: cell phones and power lines
- From: Daniel Silevitch <dmsilev@xxxxxxxxxxxx>
- Date: Thu, 28 Feb 2008 03:32:18 GMT
On 27 Feb 2008 21:41:46 -0500, Keith F. Lynch <kfl@xxxxxxxxxxxxxx> wrote:
Paul Dormer <prd@xxxxxxxxxxxxxxxxxxxx> wrote:
How high can a voltage get before the current starts arcing
to earth? Presumably you'd need taller towers. According to
Wikipedia, the highest transmission voltage is 1,150 kV in
Kazakhstan.
The breakdown voltage of air is about 3 kV per millimeter, so even at
1150 kV there's no danger of arcing to Earth if the towers are taller
than a person.
So? If that were the only determinant, your typical 100 megavolt
lightning bolt would only be about 100 feet long. Observational evidence
suggests that perhaps other factors are at work that increase the length
scales for arcing at a given voltage.
Doing a tiny bit of searching around yields, for instance,
http://en.wikipedia.org/wiki/Lightning
"The voltage depends on the length of the bolt, with the dielectric
breakdown of air being three million volts per meter; this works out to
approximately one gigavolt (one billion volts) for a 300 m (1000 ft)
lightning bolt. With an electric current of 100 kA, this gives a power
of 100 terawatts However, lightning leader development is not a simple
matter of dielectric breakdown, and the ambient electric fields required
for lightning leader propagation can be a few orders of magnitude less
then dielectric breakdown strength."
where a good chunk of the rest of the article explores the various
effects that allows propagation of an arc for distances quite a bit
farther than what would be suggested by the dielectric properties of
air.
-dms
.
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- From: Keith F. Lynch
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